When should MIS be used and when should it be disabled?
Here's a blog post from the MIS creator's announcement of the addition. He describes a little bit his goals and the purpose behind the feature. Here is also another announcement of the feature. I'll quote from the latter:


  "There are generally less samples in the same amount of time compared to vanilla Cycles, but since the samples are smarter, the resultant image is clearer."


Fewer but smarter samples could swing performance either way (as the previous two answers indicate), and the impact on performance will vary widely from scene to scene, but the overall effect is an improvement in render quality. As a general rule, when building a scene performance > quality; when rendering your final scene quality > performance. Therefore, have MIS disabled when building a scene and enable it when you do your final renders. However, since MIS may not noticeably affect performance in some scenes, and it may improve performance in others, you may still want it enabled when building your scene.

There are a number of other settings that will also impact performance and render time and all of these interact with each other in weird and unusual ways in each scene. So go ahead and play with it in different scenes and decide for yourself how and when you want to use it. CGI is a very subjective art, and only you can decide if you like how a particular setting affects your renders.